Method of and apparatus for the more efficient use of high-energy charged particles in the treatment of gasphase systems



June 30, 1959 D. R. DEWEY 1|, E1- AL 2,892,946

METHOD OF AND APPARATUS FOR THE MORE EFFICIENT USE OF HIGHENERGY CHARGEDPARTICLES IN THE TREATMENT OF GAS-PHASE SYSTEMS 2 SheetsSheet 1 FiledNOV. 25, 1955 [ll/ l FIG. 3

SE OF IN THE TREATMENT OF GAS-PHASE SYSTEMS ET AL 2,892,946

MORE EFFICIENT II 11111 1959 D. R. DEWEY ll METHOD OF AND APPARATUS FORTHE! HIGH-ENERGY CHARGED PARTICLES Filed Nov. 25, 1955 2 Sheets-Sheet 2FIG. 5

United States Patent LMETHOD OF AND APPARATUS FOR THE MORE EFFICIENT USEOF HIGH-ENERGY CHARGED PARTICLES IN THE TREATMENT OF GAS- PHASE SYSTEMSApplication November 25, 1955, Serial No. 548,877 3 Claims. (Cl.250-495) This invention relates to the bombardment of gas-phase systemsby high-energy charged particles, and in particular to a method of andapparatus for the more eificient use of such charged particles in thetreatment of such gas-phase systems. Briefly stated, our inventioncomprehends directing high-energy charged particles into the chambercontaining the gas-phase system to be bombarded, and subjecting thecharged particles to the deflecting action of a magnetic field withinthe chamber, the strength-of the magnetic field being suflicient toconfine the charged particles within the chamber. Examples of gaseousproducts include ethylene (which may be polymerized to form polyethyleneunder irradiation), methane, ethane, and propane.

The invention is particularly useful in the electron irradiation ofgas-phase systems for the purpose of causing chemical reactions such asethylene polymerization, etc., and for other purposes. Certain practicaldifliculties areencountered in the irradiation of gas-phase systems withelectrons of sufficiently high energy to permit eflicient passagethrough an electron-permeable window. Electron-permeable windows usuallycomprise thin metal foil, and electrons must have energy of at least 0.5m.e.v. for eificient passage therethrough. In the case of a 2-m.e.v.electron beam, for example, the range of the beam in gas at atmosphericpressure is of the order of 25 feet, compared with less than /z-inch forthe same beam in water or material of water density. This means, in thefirst place, that special, cumbersome and large apparatus must be usedto handle the gaseous product being irradiated if a high percentage ofthe beam is to be absorbed inthe gas, and, in the second place, that theionization density in the gas is low (i.e., there are few ion pairsproduced per cc.).

In accordance with our invention, the electron beam passes through aWindow into the gas-phase system, and shortly thereafter, before it istoo much enlarged by scattering, passes between the pole faces of amagnet (which may be in the reaction chamber or outside its walls), thismagnet having field strength suflicient to cause the electron beam tobend in a full circle, ending up in a closing spiral and therebycompleting its entire range in a very small gas volume.

' In the case of low-pressure gas-phase systems the invention isadvantageous because it reduces the resultant electron path length to avalue within the dimensions of thereaction chamber. The invention isalso advantageous'in the irradiation of high-pressure gas-phase systems,even when the pressure is sufiiciently high to provide a gas densitygreat enough to stop the electrons in a short distance, 'asa result ofthe fact that the walls of the reaction chamber in which thehigh-pressure gas-phase system is confined must be thick enough tosupport the high pressure. In order to get the electrons through thewalls of the reaction chamber or vessel containing the gas, theelectrons must be given much greater energy than that required topenetrate the relatively dense gas alone. Hence these electrons stillhave much of their energy ice after passing through the gas in thevessel, and the invention provides means for preserving this energy bycausing the electrons to stay in the gas and there to expend theirremaining energy in a useful manner.

The invention is not limited to use with electron beams, but includespositive-ion applications as well. For example, in the production ofneutrons by bombarding a gaseous isotope of hydrogen with high-energydeuterons, the invention serves to confine the deuterons within arelatively small volume of target material despite the low density ofthe latter.

The magnetic field used to confine the high-energy charged particlesshould be substantially constant during the treatment time. In general,an accelerator giving a continuous beam of charged particles, such as anelectrostatic belt-type accelerator, would require a magnetic field ofapproximately constant field strength, such as might be provided by apermanent magnet or an ironcore electromagnet. On the other hand, anaccelerator giving a pulsed beam of charged particles, such as amicrowave linear accelerator, might have a pulsed magnetic field whosefield strength is approximately constant during each pulse, such asmight be provided by an air-core electromagnet driven by a suitablepulsing circuit which is synchronized with the accelerator in someappropriate manner. Very high field strengths may be obtained in thismanner and the use of an iron core, which owing to saturation wouldunduly limit the field strength, should be avoided. Such use of a pulsedmagnetic field to produce very high field strengths is an importantfeature of the invention, for it not only facilitates the irradiation ofgas-phase systems within treatment chambers of relatively small sizewith high-energy electrons, but it also permits a beam of ions to bedelivered to a gas target in such a manner as to minimize energy loss,by confining the ions to the volume enclosed by the chamber contain--ing the gas target. In order to confine an ion beam within a givenvolume, a much larger magnetic field strength must be used than isrequired with an electron beam of equivalent energy, owing to the muchgreater mass of the ions. The use of a pulsed magnetic field in;accordance with the invention thus not only facilitates the ionbombardment of gas targets, but also enables a beam of ions to bedelivered to a very small volume of space, in the event that it shouldbe desired to concentrate the energy of the bombarding ions upon a verysmall portion of the gaseous target material.

The invention may best be understood from the following detaileddescription thereof, having reference to the accompanying drawings, inwhich:

Figure 1 is a perspective view of one embodiment of the invention;

Figure 2 is a vertical section of the apparatus of Figure 1;

Figure 3 is a diagram showing the magnetic field pattern produced by theapparatus of Figure 1;

Figure 4 is a diagrammatic view of another embodiment of the invention;and

Figure 5 is a diagrammatic view of the embodiment of the invention shownin Figure 4, but taken at right angles to the view of Figure 4. The viewof Figure 5 is not drawn to the same scale as the view of Figure 4.

Referring to the drawings, and first to Figures 1, 2 and 3 thereof, thegas-phase system to be treated is confined within a reaction chamber 1which is separated from the evacuated region of an electron accelerator2 by a suitable electron window 3 comprising, for example, a strip ofaluminuru foil. The electron accelerator 2 may be of any conventionaltype, and in Figures 1, 2 and 3 the electron accelerator 2 is providedwith means for rapidly scanning the electron beam 4 in accordance withthe teachings of U.S. Patent No. 2,602,751 to Robinson.

However, an unscanned beam may also be used, and the invention is notlimited to any particular type of electron beam.

The reaction chamber 1 may be completely closed off, or, as shown inFigure l, a suitable conduit 5 may be employed through which the gas tobe treated may be fed continuously. The reaction chamber 1 may be of anyconventional type.

In accordance with the invention, a magnetic field is created within thereaction chamber 1 by suitable means, such as the permanent magnet 6shown in Figures 1 and 3. The permanent magnet 6 is shown in Figures 1and 3 as being outside the reaction chamber 1, but the magnet mayequally well be placed inside the reaction chamber 1 and anelectromagnet, either solid-core or air-core, may be substituted for thepermanent magnet 6, all without departing from the spirit and scope ofthe invention.

Where, as in Figures 1-3, the electron beam 4 is scanned so as to enterthe reaction chamber 1 in planar form, the direction of the magneticfield in the reaction chamber lshould be perpendicular to the plane ofthe beam 4, so that the electrons are deflected as shown in Figure 2.The effect of the magnetic field is to cause the electrons to travel ina circle whose radius is proportioned to the momentum of the electrons.As the electrons collide with the gas molecules in the reaction chamber1, they lose energy and momentum with each collision, so that the radiusof the circular path travelled by each electron keeps diminishing,resulting in the spiral path shown in Figure 2.

While the embodiment of the invention shown in Figures l3 is equallywell adaptable to continuous or pulsed beams of charged particles, theuse of a pulsed beam permits one to pulse the magnetic field, therebyattaining very high field strengths. An embodiment of the inventionsuitable for use with pulsed beams of charged particles is shown inFigures 4 and 5.

Referring to said Figures 4- and 5, a charged-particle acceleratorproviding a pulsed beam is shown at 7. The invention applies to positiveas well as negative charged particles, but for illustrative purposesonly, the accelerator 7 is assumed to deliver a pulsed beam of electrons8 through an electron window 9 into a reaction chamber 10 containing thegas to be irradiated. For example, the accelerator 7 may be a microwavelinear accelerator of conventional design. In Figures 4 and 5, theelectron beam 8 is shown as unscanned, and the reaction chamber 10 issealed off. However, as previously noted, the invention is not limitedto such an arrangement.

The magnetic field is provided by a coil 11 into which the reactionchamber 10 is fitted, as shown. Alternatively, two separate coils couldbe used, one on either side of the reaction chamber 10. The coil 11 isair-cored because ferro-magnetic material would saturate at a fieldstrength less than that otherwise obtainable. One possible circuit forpulsing the coil 11 is shown in Figure 4, but other circuit arrangementswill readily suggest themselves to those skilled in the art.

In the circuit shown in Figure 4, the signal which triggers the pulsecircuit is derived from the electron beam 8 itself, which, although itpasses through the electron window 9 to a large extent, neverthelessloses some electrons to the electron window 9. The electron window 9 isinsulated from the rest of the accelerator 7 as shown at 12, and thenegative charge collected by the electron window 9 leaks oif to groundthrough a resistor 13. Thus, the arrival of a pulse at the electronwindow 9 reduces the potential of the point A. The point A is connectedto the negative terminal of a bias-voltage source 14 whose positiveterminal is connected to the cathode 15 of a thyratron tube 16 whosegrid 17 is grounded. The plate 18 of the thyratron tube 16 is connectedto one end of the coil 11, and the other end of the coil 11 is connectedto the positive terminal of a power source 19 through a resistance 20.The point B is connected to the cathode 15 via a capacitance 21.

Between pulses of the electron beam 8, the bias-voltage source 14prevents current from flowing through the thyratron 16 by keeping thecathode 15 positive with respect to the grid 17. When an electron pulsehits the electron window 9, the point A becomes negative with respect toground and the potential of the cathode 15 is lowered sufiiciently tofire the thyratron 16. This releases the electric charge stored in thecapacitance 21 and results in the delivery of a high current pulsethrough the coil 11, whereby a magnetic field of high field strength iscreated within the reaction chamber 10.

The invention is not limited to any particular type of pulsing circuit,and other types of circuits equally suitable for practicing theinvention will readily suggest themselves to persons skilled in theelectronic art. One of the important features of the invention is theprovision of magnetic fields in the treatment chamber of very highintensity through use of a pulse technique such as that just described.The higher intensity of the magnetic field permits the more eflicientuse of higher energy electrons for a given treatment chamber; or,alternatively, the more efiicient use of a smaller treatment chamber fora given electron source. The higher intensity of the magnetic field iseven more advantageous where a beam of ions is to be confined therebywithin a treatment chamber or other relatively small volume. Since thelightest ion, the proton, has a rest mass which is 1837 times the restmass of the electron, the strength of the magnetic field which isrequired to confine an ion beam of a given energy within a giventreatment chamber is at least 40 times as great as thestrength of themagnetic field which is required to confine an electron beam of the sameenergy within the same treatment chamber. Using the high magnetic fieldstrengths attainable by means of the pulse technique of the invention, abeam of ions may be delivered to a very small volume of gaseous targetmaterial.

Having thus described the method of the invention, together with severalembodiments of apparatus for practicing the method, it is to beunderstood that although specific terms are employed, they are used in ageneric and descriptive sense, and not for purposes of limitation, thescope of the invention being set forth in the following claims.

We claim:

1. Apparatus for delivering a beam of high-energy charged particles to avolume of matter of which volume the dimensions are much smaller thanthe range of said charged particles in said matter, comprising incombination: a treatment chamber bounding said volume, means forcreating such a beam of charged particles and directing the same intosaid treatment chamber, and means for creating a magnetic field withinsaid treatment chamber whose intensity in a direction perpendicular tothe direction of incidence of said beam into said treatment chamber issufiicient to confine the charged particles so that their energy isexpended substantially entirely within said treatment chamber.

2. Apparatus for delivering a beam of high-energy electrons to a voltuneof matter of which volume the dimensions are much smaller than the rangeof said electrons in said matter, comprising in combination: a treatmentchamber bounding said volume, means for creating such a beam ofelectrons and directing the same into said treatment chamber, and meansfor creating a magnetic field within said treatment chamber whoseintensity in a direction perpendicular to the direction of incidence ofsaid beam into said treatment chamber is sufficient to confine theelectrons so that their energy is expended substantially entirely withinsaid treatment chamber.

3. Apparatus for delivering a beam of high energy charged particles to avolume of matter of which volume the dimensions are much smaller thanthe range of said 5 charged particles in said matter, comprising incombination: means for creating a pulsed beam of charged particles anddirecting the same into said volume, means for creating a magnetic fieldwithin said volume whose intensity in a direction perpendicular to thedirection of incidence of said beam into said volume is sufiicient toconfine the charged particles so that their energy is expendedsubstantially entirely Within said volume, said means for creating amagnetic field including at least one coil for the creation of saidmagnetic field by the passage of current therethrough, and means fordeliver- References Cited in the file of this patent UNITED STATESPATENTS Birkeland Oct. 18,

Andriessens Apr. 8,

Slepian Oct. 11,

FOREIGN PATENTS Great Britain Oct. 30,

Great Britain Apr. 2,

